The present invention relates to a process to produce polyethylene terephthalate (PET) modified with o-phthalic acid units.
Polyethylene terephthalate, specifically when used as a raw material for making packaging materials, is modified with various comonomers in order to obtain favorable use properties. Here, preferably substances are used that break the chain orientation. This is meant to achieve a lower melting point, which is critical for the formation of by-products (such as acetic aldehyde) during production and processing, and a lower thermal crystallization rate. The lower crystallization rate allows to produce semi-finished products for different uses.
Mainly isophthalic acid (EP 465 040, U.S. Pat. Nos. 4,618,386, 4,835,247, 4,921,929, 4,940,616, 5,310,787) and cyclohexane dimethanol (U.S. Pat. Nos. 5,283,295, 5,310,787) have been described as comonomers. Both substances are special-purpose chemicals.
The use of phthalic anhydride as a catalyst in PET production (DE 2 126 217, DE 2 166 285, JP 49 013 249, JP 50 062 245, JP 52 124 098) and for modifying end groups (EP 51 553 and SU 994 490) is known. JP 08 188 919 and JP 06 184 415 describe the use of phthalic anhydride to produce polyethylene terephthalate fibers. The use of phthalic anhydride in esterification is problematic both because of the distinctive tendency of this substance to sublimate as well as because of the tendency to form cyclic oligomers with a comparatively low boiling point. This causes high losses of phthalic acid in the esterification and in the polycondensation stages.
The production of polyethylene terephthalate with a content of 25 to 55 percent by weight of phthalic acid units is patented in the Romanian patent RO 104 034. The polycondensation of bis(hydroxyethyl)terephthalate with a bis(hydroxyethyl)phthalate has been described. The phthalate is synthesized through the conversion of ethylene glycol with phthalic anhydride with a molar excess of the ethylene glycol in relation to the terephthalic acid of 3-6:1, and in the presence of zinc acetate acting as a catalyst. Phthalate produced in this manner cannot be used in the synthesis of packaging materials as the extremely high diethylene glycol content ( greater than 20%) of this ester strongly reduces the glass transition temperature of the modified polyethylene terephthalate. Also, this catalyst affects the polyester at high temperatures because zinc acetate catalyses the thermal decomposition of polyethylene terephthalate. That is why the copolyesters made in this way are exclusively used as adhesives. The synthesis of stable block copolyesters under the addition of phosphoric compounds is described in GB 1 060 401.
DE 2 706 128 patents the use of phthalic acid esters as plasticizers for polyethylene terephthalate. The esterification of the end groups of these phthalic acid esters with monofunctional alcohols (such as Nonanol) prevents the inclusion in the PET.
It is the purpose of this invention to present a process to produce polyethylene terephthalate modified with o-phthalic acid units while at the same time avoiding high phthalic acid losses during esterification and polycondensation, and while at the same time maintaining the favorable use properties provided by the phthalic acid.
To this end, a precondensate made from phthalic anhydride and ethylene glycol by way of a non-catalytic reaction is added in the esterification and/or polycondensation stages, whereby ethylene glycol is used at a molar excess relative to the phthalic anhydride of 2 to 3.5, preferably of 2.5 to 3.
As a result and according to the present invention, between 0.1 and 10 percent by weight, preferably 0.5 to 6 percent by weight, of phthalic acid can be included in the polyethylene terephthalate.
Surprisingly, it was found that the losses, which are caused by the sublimation of the phthalic anhydride and the formation of relatively highly volatile cyclic esters from phthalic acid and ethylene glycol, can be considerably minimized through the use of catalyst-free precondensates of phthalic anhydride and ethylene glycol without causing any deviation of the use properties from those of the polyethylene terephthalate modified with phthalic anhydride.
It was further found that the precondensate of phthalic anhydride and ethylene glycol should be made with an ethylene glycol excess.
It is a major advantage of the present invention that phthalic anhydride, which is available at commercial scale, is used to modify the PET.